This invention relates to computer system disk access control and more particularly to such systems where both swapping and file access capability is desired from a single disk drive (actuator).
In disk systems it is typical that the disk can transfer data faster than large dynamic memories can store it, especially when the memory is also used for execution by a host CPU. Thus, for fast bulk transfers where the maximum transfer rate is required, such as occur during swapping operations, the performane of the CPU is impacted.
For some disk transfers, however, where fast transfer rates are not imperative, it is desirable to transfer data from the disk to the host CPU memory without host CPU intervention thus enabling the CPU to execute instructions faster. Thus, in prior systems it has become customary to use two disk drives (actuators), one arranged in an efficient format for bulk transfers at a high transfer rate and the other arranged in a different format efficient to transfer disk data without host CPU intervention at a lower transfer rate. These prior arrangements have the disadvantage of requiring two disk drives (actuators), even when the system would not otherwise require such duplication.
The use of two disk drives, however, allows the user to physically separate the "file" area (lower transfer rate) from the "swap" area (higher transfer rate), thereby allowing file accesses and swaps to be overlapped. For example, if only one disk drive is used, a swap cannot take place until a file access on that drive is completed. With two disk drives, on the other hand, on disk can be transferring data while the head of the other disk is being positioned to the desired location, thus reducing the head positioning delay.
In small systems, where both low cost and high performance are imperative, two disk drives and a separate direct memory access (DMA) controller would raise the cost to an unacceptable level. Accordingly, a one disk solution where both types of transfers can occur economically is needed.